1. Field of the Invention
This invention relates generally to the field of geophysics and more particularly to a method for estimating the travel path of a seismic signal through the earth""s subsurface.
2. Description of Related Art
Knowledge of the subsurface structure of the earth is useful for finding and extracting mineral resources such as oil and natural gas. Geophysical techniques which have been utilized for obtaining knowledge of the subsurface include surface seismic exploration, vertical seismic profiling and crosswell seismic tomography.
Surface seismic exploration produces data which cover a large volume of the earth""s subsurface, however, data resolution is low. The maximum utilizable seismic frequencies are several hundred Hz., and the resulting spatial resolution is correspondingly limited. Crosswell seismic imaging provides data for the earth""s subsurface extending between well locations, and it provides this data at a higher resolution than is provided by surface seismic data. Vertical seismic profiling typically employs a seismic source at the earth""s surface and seismic receivers in a borehole. However, xe2x80x9creversexe2x80x9d vertical seismic profiling employs a source in the borehole and receivers on the surface. Typically, data resolution provided by vertical seismic profiling is also better than for surface seismic operations.
Conventional crosswell seismic imaging typically utilizes a pair of boreholes in proximity to a reservoir of interest. In the first of these boreholes, a seismic source is deployed for emitting seismic energy into the region of interest, often as a swept frequency signal extending through a selected frequency range. The source is sequentially moved through a series of positions within the first borehole and a seismic signal is generated at each position. The seismic energy passes through the subterranean formation of interest to the second one of the pair of boreholes. A receiver array is typically deployed within the second borehole and, like the seismic source, the receiver array is moved through a series of positions within the second borehole. By transmitting a signal from each source position in the first borehole and receiving data from each source position at each receiver position in the second borehole, a seismic crosswell data set is generated. Surveys may also be conducted across a region penetrated by a plurality of boreholes by deploying a source in one of the boreholes and deploying receivers in each of a plurality of boreholes so as to simultaneously record a plurality of data sets.
The data records from a typical crosswell survey represent a very large body of information. For example, if data are obtained from three hundred different receiver positions and each receiver position receives data from each of three hundred source positions, the result will be ninety thousand separate data records or xe2x80x9ctracesxe2x80x9d. Crosswell imaging contemplates the use of this data to produce a map representing a seismic parameter, such as velocity, of the subsurface structure in the vicinity of the boreholes.
In general, the subsurface structure is mathematically modeled and this model is used as a basis for forming a tomographic image of a seismic parameter of interest, such as velocity. In one model which is typically used, the vertical plane extending between two boreholes is divided into square pixels and the region within a pixel is assumed to be homogeneous with regard to seismic properties such as wave propagation velocity. A system of equations is set up, based on the travel times and travel paths of the crosswell signals between source and receiver locations. This system of equations is then solved to determine the velocity profile within the subsurface structure between the boreholes.
Another method utilizes geological formation boundaries, such as formation tops, which have been identified from well logging data, or other data, as a basis for forming the model, which may be a three dimensional model. Data representing the identified formation tops are applied to 2-D Chebyshev polynomials, and subterranean surfaces are then defined which approximate the interfaces between subsurface strata of differing lithology. Additional surfaces, extending laterally between the surfaces calculated from the formation tops, are then added to the model. A seismic property of interest, such as velocity, of the subsurface region between the surfaces is then modeled with another Chebyshev polynomial in each layer. A system of equations is then set up, based on the travel times and travel paths of the crosswell signals between source and receiver locations. This system of equations is solved to determine the velocity profile within the subsurface structure between the boreholes in a manner which is substantially analogous to the method utilized with the pixel based model.
Regardless of whether the data is surface seismic data, crosswell seismic data or vertical seismic profiling data, an estimation of the travel time and the travel path from source to receiver locations is required for processing the data. Prior art techniques have included the shooting method and the ray bending method. It is an object of this invention to provide an improved method for estimating such travel times and travel paths.
In one embodiment, the invention includes a method for estimating seismic signal propagation raypaths from seismic source locations through a subsurface formation to seismic receiver locations. Raypaths are determined which minimize the value of an expression which is a function of travel time and distance between a source location and a receiver location. The value of the expression depends on the value of a parameter which balances the weighting of travel time and distance in the expression.
In another embodiment, the invention includes a method for estimating the travel path of a seismic signal from a seismic source location through a subsurface formation to a seismic receiver location, in which a value for a parameter is determined and a travel path is estimated which minimizes the value of a mathematical expression comprising a function of said parameter, travel time and distance. The parameter balances the weighting of travel time and distance in the expression. The value of the parameter is determined so that the estimated travel path for the seismic signal has a travel time substantially equal to TSnell+1/(2f ) when the value of the expression is minimized, where:
TSnell=the Snell""s Law raypath travel time from said source location to said receiver location; and
f=the dominant frequency in said seismic signal.